The "oManual" (Open Manual) specification is an interesting development that's worth keeping an eye on, although I didn't find it sufficient for my own work. It is a far measure more simple than DITA, which is a plus, and it has O'Reilly Media as one of the founding contributors. (O'Reilly is my publisher.) Perhaps its most interesting feature is that it is both a schema and an API, which could allow onscreen manuals to finally become a lot smarter.

On the downside, probably due to its origins with iFixit, it is heavily biased towards repair and assembly procedures. It also has some in-built appearance controls, which is surprising in this culture that insists on complete separation of "content" and format.

it appears the only big client so far is the Dozuki hosted wiki, based on the project's website, but the unfortunate side effect of the name is that it's difficult to search for other sites with more to say about it.